The 2010 Senate elections, though still 18 months off, have entered a new and critical phase in recent days with big names joining and dropping out of key contests, and national political leaders exerting their influence on the races in bold and unprecedented ways.

From Florida Gov. Charlie Crist’s decision to enter the GOP primary to President Barack Obama pushing Rep. Steve Israel (D) out of the New York primary, the parties are now fully engaged in shaping the Senate battlefield. When the history of the 2010 elections is written, the past week may be considered a watershed.

Recent developments have also shown how critical primaries

will be in determining the ultimate Senate playing field — and how internecine warfare continues to vex both parties in critical states. The ability of party leaders to finesse these potential minefields could go a long way in determining the Senate lineup in the 112th Congress.

Republicans were crowing in the wake of the recent headlines.

“Last week one of the most popular governors in the country jumped in the Florida Senate race,— said Brian Walsh, a spokesman for the National Republican Senatorial Committee. “The Democrats’ No. 1 recruit for Senate in North Carolina dropped out. Democrats continue to have a contentious primary on their hands in Ohio and they are unable to figure out who they want to run in Illinois. This was one of the worst weeks for the Democrats this cycle to date, and all the spin in the world can’t disguise that.—

But Democrats also took heart from some of the news. As they approach 60 seats pending the outcome of the disputed Minnesota race from last cycle, they are content in the knowledge that the 2010 playing field appears to tilt in their favor for the third straight election cycle.

Senate Democrats have credited their 14-seat pickup in the past two election cycles in part to their success recruiting the strongest possible candidates in key races and muscling aside potentially problematic primary opponents. NRSC Chairman John Cornyn (Texas) clearly had the same idea in mind when he quickly endorsed Crist last week despite the presence of former Florida Speaker Marco Rubio in the GOP primary.

Although polls show Crist with a formidable lead in hypothetical general election matchups, some conservative groups, including the Club for Growth, have been critical of his performance in Tallahassee and are promoting Rubio.

Republican National Committee Chairman Michael Steele vowed to remain neutral in the primary during an appearance on NBC’s “Meet the Press— on Sunday. Although Steele did not say so on the program, Rubio is the former Florida chairman of GOPAC, a political organization that Steele led before taking over the RNC.

The leading Democrat in the Florida Senate race is Rep. Kendrick Meek, though state Sen. Dan Gelber is also running.

“We’re all anxious to see the results of the impending battle royal in Florida since the last time national Republicans bucked the Club for Growth, their candidate left the party,— Democratic Senatorial Campaign Committee spokesman Eric Schultz said on Monday, referring to Pennsylvania Sen. Arlen Specter.

Specter announced late last month that he would run for re-election as a Democrat in 2010 because he did not think he could beat ex-Rep. Pat Toomey, a Club for Growth favorite, in the Republican primary. Now Toomey is the lone GOP candidate in the Senate race, but party leaders are hoping to find someone with more widespread appeal for the general election, and are talking to Rep. Jim Gerlach, among other potential contenders.

But Pennsylvania could be fraught with peril for Democrats as well. As soon as Specter announced his party switch, Obama and Vice President Joseph Biden pledged to support him next year. But Specter has confounded Democratic leaders with some of his votes and public pronouncements since then, and former National Constitution Center President Joe Torsella (D), an ally of Gov. Ed Rendell (D) who had been campaigning for Senate for months, was chased out of the race only last week.

Torsella aides insisted that no one pressured him to drop out.

The bigger question now is whether Rep. Joe Sestak (D) decides to run against Specter — and whether Obama can persuade the Congressman to stay out of the Senate race the way he did with Israel in New York.

“I’m still very strongly considering getting in,— Sestak said just before Torsella’s announcement. “And actually, there has been a pleasant reaction both inside my district and outside my district from individuals.—

Sestak also said that no party leader has asked him to stay out of the race.

Although Sestak is not as well-known as Specter, his bank account rivals that of the state’s senior Senator — the Congressman had $3.3 million as of March 31.

Sestak could also get a boost from labor unions, which continue to be the dominating force in Pennsylvania Democratic primaries. Specter spurned his union supporters earlier this year when he announced that he would vote against cloture on the labor-backed Employee Free Choice Act — a reversal of his 2007 vote on the same bill. If union groups get behind Sestak, who is a vocal supporter of EFCA, he could wage a vigorous primary challenge against Specter and give national Democrats a primary headache in the Keystone State.

Obama may also be under pressure to mold the Senate field in Illinois, where his old Senate seat is up for grabs in 2010 and mystery still surrounds the intentions of beleaguered appointed Sen. Roland Burris (D).

As the playing fields dwindle in other states, the number of potential Democratic Senate candidates in Illinois only appears to grow with the news that Attorney General Lisa Madigan is reconsidering a bid for Senate. Madigan could clear the Democratic primary field if she decided to run for Senate; however, most Illinois political operatives believe she will run for governor in 2010.

State Treasurer Alexi Giannoulias (D) is the only announced candidate in the race so far, but Rep. Jan Schakowsky (D) said she will decide by mid-June whether to run for Senate. Burris has not said yet whether he will run for re-election, but he could start a campaign in an effort to pay off some of his mounting legal bills. Finally, businessman Chris Kennedy (D) is also considering running for the seat and could have the backing of many Chicago business leaders.

Rep. Mark Kirk remains the only Republican to publicly talk about running for Senate. Public polling data has shown Kirk could be in a position to defeat any of the Democrats except Madigan, so Kirk is waiting and watching the Democratic field before making a decision.

Despite Obama persuading Israel to abandon his plans to challenge Sen. Kirsten Gillibrand (D-N.Y.) next year, the appointed Senator is not completely home free when it comes to avoiding a primary. In fact, a primary seems almost inevitable; the question is how competitive it will be.

Reps. Carolyn McCarthy (D) and Carolyn Maloney (D) continue to ponder the race, though both have been mum about their political plans since Israel’s announcement late Friday. Manhattan Borough President Scott Stringer (D) and Suffolk County Legislator Jon Cooper (D) — neither of whom has to sacrifice his job to run next year — have both created exploratory committees for possible Senate bids.

But there is increasing momentum behind Gillibrand. DSCC Chairman Bob Menendez (N.J.) stopped short of an endorsement Friday, but he said the DSCC “looks forward to continuing working with her to make sure she well-prepared for her race.— And on Monday, Reps. Brian Higgins (D) and Michael McMahon (D), as well as state Senate Majority Leader Malcolm Smith (D), the second-most-powerful African-American in state government, endorsed Gillibrand.

Although Democratic leaders have made their preference known in Ohio, Secretary of State Jennifer Brunner has insisted she is going to stay in the Senate race against Lt. Gov. Lee Fisher, the party favorite.

Fisher routed Brunner in their first quarter of fundraising, bringing in more than $1 million to her $207,000. With support from popular Gov. Ted Strickland (D) and some members of the Congressional delegation, Fisher is undoubtedly the Democratic frontrunner in the race to replace retiring Sen. George Voinovich (R).

But EMILY’s List could wade into the race, sending much-needed financial resources in Brunner’s direction. EMILY’s List continues to dedicate political staff and resources to her operation, but it has not endorsed her yet.

While Senate Democrats fret over the looming primary, Republicans escaped an intraparty battle last week when state Auditor Mary Taylor announced she would run for re-election instead of the Senate. Her announcement leaves the field almost completely clear for former Rep. Rob Portman (R) to raise funds while Democrats fight it out.

The parties’ circumstances are flipped in Missouri, where the GOP race to replace retiring Sen. Kit Bond (R) could become a nasty three-way affair. Last week saw the emergence of Thomas Schweich, a former special ambassador to Afghanistan, who is being touted by popular former Sen. Jack Danforth (R). Already Rep. Roy Blunt is running with the support of the national GOP establishment, and state Treasurer Sarah Steelman has not ruled out a bid.

Democrats have coalesced behind Secretary of State Robin Carnahan, and a new poll last week in the Show Me State showed her with a substantial lead over both Steelman and Blunt.

Democrats are still waiting for a top-tier candidate to emerge in North Carolina, where state Attorney General Roy Cooper (D) announced last week he would not run against Sen. Richard Burr (R). But Burr is by no means a lock for re-election in a state that appears to be trending Democratic; Sen. Kay Hagan (D) was hardly the first choice of party leaders when she launched her 2008 bid.

In Kentucky, the Republicans’ problem is their incumbent, Sen. Jim Bunning (R). Party leaders are convinced that their chances of holding the seat would increase dramatically if the 77-year-old Senator didn’t seek a third term, but so far he has stubbornly refused to take the overt hint. Kentucky Secretary of State Trey Grayson (R) has created an exploratory committee for a possible Senate run, but he has insisted that he won’t challenge Bunning.

The Democrats have a tough Senate primary of their own between two top-tier candidates, Lt. Gov. Dan Mongiardo and state Attorney General Jack Conway, but for now party leaders seem content to let the primary play itself out and watch the drama on the Republican side.

John McArdle and Lauren W. Whittington contributed to this report.