Correction Appended

Senate Republicans are slowly starting to open their wallets to fuel Sen. John McCain’s presidential bid, but only about a third of the Arizonan’s colleagues have come through so far.

According to Federal Election Commission filings through March 31, 18 Senators — 17 Republicans and one Independent Democrat, Joe Lieberman (Conn.) — had written a check to support McCain’s 2008 bid. Most of those GOP Senators contributed $5,000, the maximum amount permitted for a leadership political action committee.

McCain officials say more Senate contributions have come in since the close of the first quarter of the year. While not yet publicly available, one of those noteworthy donations came from Minority Leader Mitch McConnell (R-Ky.), who gave $5,000 from his Bluegrass Committee, they said.

Sources close to McCain say the Senator turned presidential hopeful has made collecting funds from his fellow Republicans a priority and has set a goal of winning financial support from all 48 of his colleagues. McCain’s aides say that while a share of Republican lawmakers have yet to pony up, they have received commitments from nearly all of them and none have refused to give.

“We’ve seen a lot of goodwill,” said a source close to the McCain camp.

McCain secured the title of the presumptive GOP nominee in early March, and since then he has been putting together a general election campaign apparatus that includes coordination with Congressional Republicans on message and strategy. Campaign officials also have spent recent weeks making personal appeals to lawmakers for fundraising help, an effort that’s been reinforced by a little GOP leadership peer pressure.

In mid-March, Sen. Kay Bailey Hutchison (Texas), the GOP Policy Committee chairwoman, and House Minority Leader John Boehner (Ohio) penned separate letters to their respective conferences to urge lawmakers to give McCain money. Both Republican leaders, who have already maxed out to McCain, made the case that the Arizona Senator is a strong conservative and an American hero who deserves united party backing heading into November.

“Help send a strong signal of support from the House Republican Conference and make a contribution to Senator McCain from your leadership PAC or campaign committee today!” Boehner wrote on March 12.

In addition to the Senators who have contributed, some 21 House Republicans had given to McCain as of the end of March.

McCain’s campaign and his allies say they expect the pace of the Member contributions to pick up as the general election nears, particularly as McCain barnstorms the country and holds events on behalf of and in concert with other GOP Senators and House Members. Also, McCain is anticipating a bloc of GOP lawmakers –– particularly Senators –– will open their wallets during a big-ticket fundraising gala in Washington on May 15.

“Candidly, we’ve been really fortunate,” the McCain source said. “Everyone has come on board who was asked.”

“Some people like to do it when we come to their states,” this source added. “That’s why we haven’t maxed out yet from all 48 Senators.”

Still, McCain appears to be making inroads within the party, especially among those conservatives who swore off supporting him in the early days of his campaign. Even some of his longtime GOP Senate colleagues, who were initially leery and even publicly critical of his candidacy, have come around.

Sen. Thad Cochran (R-Miss.), who supported presidential contenders ex-Sen. Fred Thompson (Tenn.) and then former Massachusetts Gov. Mitt Romney (Mass.) before finally endorsing McCain, said last week that McCain is “our candidate” and as such he is going to do his part to help bolster the campaign. Cochran gave $5,000 from his leadership PAC to McCain in March.

“I think you’ll find Republican officeholders across the country, and particularly in the Congress, rallying to support him and help him win the election,” said Cochran, a veteran appropriator who in January told the Boston Globe that the thought of a McCain presidency “sends a cold chill down my spine.”

Another one-time Thompson backer, Sen. Bob Corker (Tenn.) agreed, saying that while he has yet to make his McCain contribution, he plans to do it soon. Corker, a freshman, said he is in the process of assembling his leadership PAC and that his first priority will be writing checks to his threatened GOP colleagues.

“I’m sure I will,” Corker said. “It’s no signal. We are just trying to focus on the guys who really need to show strong numbers. I will continue to do as much as I can to help, including Sen. McCain.”

This fall, 23 GOP-held seats are up for grabs, compared with just a dozen Democratic seats. And as of March 31, Democrats had $37.8 million in the bank compared with the GOP’s $17.3 million.

House Republicans are in tougher shape with $7.2 million on hand, compared with the House Democrats’ $44.3 million.

Sen. Lindsey Graham (S.C.), McCain’s closest Senate confidant, said “there’s been overwhelming support for John from Congress.” Graham said he hasn’t had to do much direct lobbying for money, adding that everyone in his GOP Conference has either contributed or has “shown a willingness to do so.”

He said that beyond adding to McCain’s bottom line, contributions from Senators is a critical indicator of McCain’s base of support among those who know him best. Graham said that “it’s important to the party and important to the challenge we face. But also it’s important so that we can compete against the Democrats.”

In addition to Graham, Cochran, Hutchison, Lieberman and McConnell, other McCain contributors include Sens. Jon Kyl (Ariz.), John Thune (S.D.), Wayne Allard (Colo.). John Sununu (N.H.), Lamar Alexander (Tenn.), Mel Martinez (Fla.), John Ensign (Nev.), George Voinovich (Ohio), Tom Coburn (Okla.), John Warner (Va.), Susan Collins (Maine), Richard Burr (N.C.), Gordon Smith (Ore.) and Olympia Snowe (R-Maine). Some of them are longtime friends of McCain and his earliest backers for the White House.

The McCain source said that while the money is picking up for the presumptive Republican nominee, there isn’t a formal “structure or sanctioned event” in place to solicit the Senator or House Member contributions. This source said McCain officials have tried to make their plea, while remaining cognizant of lawmakers’ priorities, including their own re-election needs.

“If they have the wherewithal to give, we believe they will give,” the source said. “We feel very confident about that.”

McCain can use the fundraising help. He lags behind the Democratic contenders, Sens. Barack Obama (Ill.) and Hillary Rodham Clinton (N.Y.), each of whom has raised more than $100 million. As of March 31, McCain had raised $81 million and had $12 million on hand.

Sen. Johnny Isakson (R-Ga.), who endorsed McCain early on but has yet to make a contribution, said Republicans will step up: “He’s our man. We’re all in this thing together.”

Correction: May 6, 2008

The article omitted the name of Sen. Olympia Snowe (R-Maine) from a list of Senators who have contributed to the presidential campaign of Sen. John McCain (R-Ariz.). Snowe gave McCain’s campaign $2,000 from her re-election account and personally contributed an additional $2,300.