The prospect of a Republican Senate majority relies more than ever on a strong showing at the top of the ticket by GOP presidential nominee Mitt Romney.

With less than eight weeks to go and both national party conventions complete, a path remains for the Republicans after several weeks of shifts in the Senate landscape. But for the GOP, it’s now more akin to a game of hopscotch than a straight-line sprint to the majority.

It could require a leap over Missouri – once a virtual sure thing for the GOP – to Connecticut, where the Senate race appears closer than it ever was in 2010. Overall, the party’s hopes hinge on a baseline of three races in Republican-leaning states, a couple of states President Barack Obama should carry by a significant margin and a few close contests in presidential swing states – where Romney’s performance especially matters.

“It’s one of those situations where there are enough close races in swing states that I think whoever wins the presidency will probably win the Senate,” said Dan Judy, a Republican pollster at North Star Opinion Research.

That was not necessarily the case a month ago, when Missouri still appeared to be a likely pickup for the party and New Mexico remained a margin-of-error race. Since then, a plethora of polling showed Rep. Martin Heinrich (D) taking a solid lead over former Rep. Heather Wilson (R), and both national Senate committees pulled money out of that state. In Missouri, Rep. Todd Akin’s remarks about “legitimate rape” potentially doomed the GOP’s chances of defeating the most vulnerable Democratic incumbent of the cycle, Sen. Claire McCaskill.

With Missouri’s status shifting and other developments, Democrats are more confident in their ability to hold on to the majority Nov. 6.

“It’s far from in the bag, but it definitely looks better than it did about a month ago,” said Mike Gehrke, a Democratic pollster at Benenson Strategy Group. “The races that were close then are close now, they just look better for us now.”

Republicans must win four seats, net, to reclaim the majority. But the loss of any they currently hold would in turn increase the number of Democratic-held seats they must flip to assume control of the chamber. The most likely casualty at this point is the seat of retiring Sen. Olympia Snowe in Maine. Former Gov. Angus King (I) is favored and likely to caucus with Democrats if elected.  

In an effort to salvage the seat, the National Republican Senatorial Committee placed a $600,000 media buy for 2,000 gross ratings points for the next two weeks. The party’s chances there lie in the makeup of the contest. With a three-way race, Secretary of State Charlie Summers (R) doesn’t need 50 percent to win, but he needs state Sen. Cynthia Dill (D) to take a significant portion of the Democratic vote.

The next challenges for Republicans are defending seats in two tossup races: Nevada Sen. Dean Heller (R), appointed by Gov. Brian Sandoval (R) earlier this cycle, against Rep. Shelley Berkley (D); and Massachusetts Sen. Scott Brown (R) against Harvard professor Elizabeth Warren (D). Obama is favored in both states, but in the Bay State to a far greater degree. Other races to watch are for GOP open seats in Arizona and Indiana.

In the Hoosier State, Democrats see a chance for Rep. Joe Donnelly (D) against state Treasurer Richard Mourdock (R), as Democratic outside groups have recently hit the airwaves on Donnelly’s behalf.

The Republicans’ absolute bottom line path to the majority includes three states Obama won’t win. The GOP must defeat Montana Sen. Jon Tester and pick up open, Democratic-held seats in Nebraska and North Dakota. If the GOP misses on any of those, a Republican majority would be highly unlikely.

All three are winnable propositions for Republicans, but two of them are pure tossups. Strategists in both parties have complimented the smart campaign Tester has run against Rep. Denny Rehberg (R), who’s won six straight statewide federal races. Meanwhile, former North Dakota Attorney General Heidi Heitkamp (D) has consistently held a small lead or been tied with Rep. Rick Berg (R) in internal Democratic polling.

If Republicans are successful in those three states, as the party believes it will be, focus then turns to two presidential battlegrounds: the Wisconsin race between Rep. Tammy Baldwin (D) and former Gov. Tommy Thompson (R) and the Virginia battle between former Govs. Tim Kaine (D) and George Allen (R). Should McCaskill triumph and the GOP lose only Maine, taking the majority would require Republicans to win both.

Republicans are optimistic in Virginia and Wisconsin, the latter a hotbed of partisan politics for the past year. However, coming off a late primary, Thompson just got his first hit from national Democrats. The Democratic Senatorial Campaign Committee launched its opening salvo in the Badger State on Wednesday, the start of a $5.2 million commitment to help keep the seat of retiring Sen. Herb Kohl (D).

Virginia has been one of the top targets of spending at both the presidential and Senate levels, including a $1 million ad campaign launched against Kaine by Crossroads GPS on Tuesday. As of last week, the DSCC had $8.1 million in fall TV time reserved in Virginia, and the NRSC had about $5.5 million reserved. Observers on both sides believe it’s unlikely Allen will win if Romney doesn’t.

A win in Connecticut would be a downright coup for Republicans, who haven’t won a Senate seat in the state since 1982. Using money saved in New Mexico, the DSCC went on the air in the Nutmeg State on Wednesday, spending about $320,000 on behalf of Rep. Christopher Murphy (D). He’s fending off a challenge for the seat of retiring Sen. Joe Lieberman (I) from former WWE CEO Linda McMahon (R), who faces a steep climb with Obama atop the ballot.

The Republican, who lost to now-Sen. Richard Blumenthal (D) in 2010, has had a presence on the air in the New York City market. She’s downplayed her party identification and successfully petitioned to appear on both the Republican and
Independent lines on the ballot, a tactic that could earn her a chunk of voters unwilling to pick a Republican.

Along with Virginia, Wisconsin and even Nevada, a Romney victory – or at least a strong showing – would significantly increase Republicans’ chances against Florida Sen. Bill Nelson (D) and Ohio Sen. Sherrod Brown (D). Both incumbents remain favored at this point in the race.

“My sense is that it’s definitely still possible for the Republicans to take the majority and maybe likely that they get to 50,” Judy said. “The flip side of that is I think they’re going to need Romney to win to even get to 50 or 51. I think they’re going to need him to carry across the finish line a couple of candidates in the swing states.”