Mitt Romney has a head start on his Republican rivals in garnering endorsements for his fledgling presidential campaign on Capitol Hill, and he is set to announce another big-name backer today when former Speaker Dennis Hastert (R-Ill.) is expected to line up behind the former Massachusetts governor.

Unlike Romney, most candidates have yet to name official Congressional liaisons for their campaigns, with four White House hopefuls officially entering the fray just within the past week.

All total, six Senators and four House Members are exploring a presidential run, with Sens. Hillary Rodham Clinton (D-N.Y.), Barack Obama (D-Ill.) and John McCain (R-Ariz.) the most prominent among them.

Romney has been the most aggressive by far in working to line up early Congressional backers, in part because he has no natural ties to Capitol Hill but also to help quell concerns among some party activists about the fact that he is a Mormon.

Romney named Rep. Jim McCrery (R-La.) to lead his outreach effort to House Members, with Reps. Howard McKeon (R-Calif.) and Dave Camp (R-Mich.) assisting in the effort as well.

“These folks are critical because they each have their organizations, which are going to help in a lot of the key states,” said a source within the Romney camp. “Be it financial, be it the ability to serve as a surrogate to their constituents, and grass-roots organizing as well.”

Rep. Jack Kingston (R-Ga.) noted the broad geography represented by Romney’s House team and said that such early Congressional endorsements can help build momentum for a candidate.

Associates of Sen. John McCain (R-Ariz.), a frontrunner for the GOP nod, say the campaign isn’t fazed by Romney’s early announced support. They tout his lengthy experience on Capitol Hill and argue that McCain may well be his own best liaison.

“Certainly the Senator has very strong relations with Members of Congress and there is quite a bit of private discussions taking place and announcements will come at an appropriate time,” said one source within the Senator’s camp.

Kingston, who is “clearly leaning Romney” at this point, said Members can be helpful to presidential candidates not only by serving as a surrogate to their constituents but also in identifying large donors.

“Remember, the primaries are always inside party baseball to begin with,” he said. “If they [Members] have a comfort level with the candidate, that can be shown to their constituents back home.”

Kingston recalled that former Rep. Rob Portman (R-Ohio) was an instrumental in helping then-Gov. George W. Bush (R) with fundraising for his 2000 presidential campaign. Likewise, Rep. Roy Blunt (R-Mo.) served as the Bush campaign’s House liaison, a post that initially was offered to Rep. John Boehner (R-Ohio), now the Minority Leader.

Boehner turned the initial offer down because he already was committed to the presidential campaign of his home-state colleague, then-Rep. John Kasich (R-Ohio).

The 2008 primaries, more so than recent White House elections, will test Member loyalties on both sides of the aisle.

This cycle, House Democrats seem to face some of the biggest quandaries with the candidacies of both Clinton and Obama, who would be the first woman and the first black president, respectively, if elected.

Rep. Rahm Emanuel (D-Ill.) has come under pressure from both campaigns to commit his support. Obama is a fellow Chicagoan, but he got his political start in the campaign and administration of President Bill Clinton and his endorsement of Sen. Clinton had long been expected before Obama entered the fray.

In late 2003 and early 2004, several House Members came out in support of former Vermont Gov. Howard Dean (D), whose campaign surged to frontrunner status with the support of Web-based party activists only to flame out after votes were cast in Iowa and New Hampshire.

But how much Member endorsements ultimately matter is debatable.

One GOP strategist not aligned with any of the campaigns noted that while Congressional support can be helpful, securing endorsements from governors, mayors and other local elected officials with a sophisticated political apparatus often are more of a priority.

“It’s not going to make or break your campaign,” the strategist said of Members’ support. “But it certainly helps in terms of breaking into their networks, especially when it comes to fundraising.”

Clinton, simply by virtue of being from such a big state, is expected to be able to chalk up a significant number of early Congressional endorsements.

Sen. Charles Schumer (D-N.Y.) already is publicly supporting his fellow Empire State Senator, and the state has 23 Democratic House Members, most of whom are expected to back Clinton enthusiastically.

Many in Clinton’s inner circle are veterans of New York politics, including Howard Wolfson, a former top aide to Rep. Nita Lowey (D-N.Y.).

House Ways and Means Chairman Rep. Charlie Rangel (D-N.Y.) — who helped persuade Clinton to run for the Senate in the first place — has told New York reporters that he feels somewhat torn, because he urged Obama to run for president, but most New York insiders expect him to eventually side with Clinton.

In the case of Connecticut Sen. Chris Dodd, Rep. Rosa DeLauro (D-Conn.) is expected to play a prominent role in reaching out to Members.

“We expect that Dodd is going to get strong support from the House, because it’s a body he served in before,” a DeLauro spokeswoman said.

DeLauro’s husband is Democratic pollster Stan Greenberg, a top adviser to Dodd’s campaign. Freshman Rep. Christopher Murphy (D-Conn.) also recently endorsed the Senator.

Former Sen. John Edwards (D-N.C.) has a strong connection to the House in former Rep. David Bonior (D-Mich.), who has signed on to manage his campaign.

Bonior, who served 12 years as House Democratic whip, has close ties to organized labor groups, but he has also been reaching out to his former colleagues.

In an interview Monday, Bonior estimated that Edwards has about two to three dozen Members who are supporting his candidacy, although he declined to make those names public.

Former Rep. Max Sandlin (D-Texas) and Rep. Albert Wynn (D-Md.) have been among those leading the effort to whip support. Edwards also is expected to have strong support from the North Carolina delegation.

“We have been reaching out to people for a while now,” Bonior said.

He said that many Members ultimately choose to keep their powder dry early on and that Members should not rush to make decisions now.

“All of that’s going to play itself out over the next year and year a half,” he said.

Obama, who officially entered the fray last week, does not have any sort of official liaison at this point.

Rep. Artur Davis (D-Ala.) was the first Member to come out and endorse his candidacy and the Alabamian is expected to play a role in helping garner support for his bid among his colleagues.

Rep. Jan Schakowsky (D-Ill.) has offered to whip support and introduce Obama to her colleagues in coming weeks.

“The most likely scenario is to have a breakfast, just to introduce him to our Members,” she said Monday. “A lot of my colleagues don’t know him.”

She said Congressional endorsements do matter.

“Every Member of Congress is a leader in his or her district and has a constituency that often looks to us for guidance when it comes to endorsing candidates,” Schakowsky said.

A candidate such as New Mexico Gov. Bill Richardson (D) doesn’t have many in-state Members to turn to, as the state’s Congressional delegation has just two Democrats: Sen. Jeff Bingaman and Rep. Tom Udall.

Bingaman already has endorsed Richardson and Marissa Padilla, Udall’s spokeswoman, said the Congressman hopes to serve as Richardson’s Congressional liaison.

“He’s endorsed him, urged him to run prior to his decision, and will do anything he can to help him reach out to other Members of Congress,” Padilla said.

But the governor and Udall have a complex relationship. The two were rivals for the House seat Richardson won in 1982 — and Udall now holds.

Richardson also has a wealth of Congressional contacts from his 15 years in the House, and he also could benefit from support from Members of the Congressional Hispanic Caucus.

Similarly, former Iowa Gov. Tom Vilsack (D) has won the backing of Sen. Tom Harkin (D-Iowa), who is taking the lead on coordinating outreach to Members on Capitol Hill.

“He is obviously a particularly persuasive advocate on behalf of Gov. Vilsack,” said campaign spokesman Josh Earnest.

Among Republicans, Sen. Sam Brownback’s (R-Kan.) logical emissaries in the House would be social conservatives, although Romney’s selection of Rep. Tom Feeney (R-Fla.) to do outreach to conservatives shows that the Kansas Senator may have to work for those endorsements.

Former New York Mayor Rudy Giuliani (R) has signed up former Rep. Jim Nussle (R-Iowa) to be a senior consultant to his exploratory effort. Nussle just left Congress at the end of last year and surely will play a leading role in reaching out to current Members.

Giuliani is likely to look to the New York delegation as a starting point for collecting Congressional support, unless former New York Gov. George Pataki (R) decides to toss his hat into the race. A spokesman for Rep. Peter King (R-N.Y.), said his boss is “leaning toward” backing the former mayor, should he run.

Giuliani has been a top campaigner and fundraiser for Republicans since leaving office and could seek to cash in some of the chits he’s collected from Members he’s helped elect and re-elect. Still, his campaign has appeared to be less focused on outreach to Members than Romney and McCain.

McCain’s most vocal supporter is Sen. Lindsey Graham (R-S.C.). In the House, his supporters include home-state Reps. John Shadegg, Rick Renzi and Jeff Flake.

He has not named a liaison in the House.

“Sen. McCain enjoys many friendships in the nation’s capital and has numerous leaders in government reaching out on his behalf,” said McCain campaign spokesman Danny Diaz.

McCain, like Giuliani, has been a megastar on the campaign trail and also garnered the goodwill of some House conservatives when he backed Shadegg in last year’s race to be minority leader against Boehner.

Josh Kurtz contributed to this report