With campaign dollars in short supply as the presidential candidates head into Super Tuesday, supporters of Sen. John McCain (R-Ariz.) and Mitt Romney are trying to shake the money tree and lure former Sen. Fred Thompson’s K Street backers to their team.

Many expected Thompson to endorse McCain almost immediately after he withdrew from the Republican presidential race on Jan. 22 — and assumed that his lobbyist supporters would follow suit.

But Thompson’s decision to delay an endorsement has caused many of his former lobbyist supporters to delay switching their endorsements as well, leaving them open to entreaties from other candidates.

Yet many lobbyists say there’s a decided ground swell of Thompson supporters either openly or quietly, behind the scenes, who are backing McCain.

“We’ve been working [the phones] since the day Thompson announced he was out of the race,” said John Green, of Ogilvy Government Relations, a McCain supporter who has been calling Thompson lobbyists. “The bottom line is if there is more Washington money for the campaign it would all go straight to Super Tuesday. One dollar is more than a dollar we didn’t have.”

That thinking has McCain supporters throwing a last-minute fundraiser tonight at Charlie Palmer Steak. The 5:30 event — $1,000 per person, $2,300 per political action committee — was organized in a little less than a week and is expected to raise more than $300,000 for the Senator.

And organizers are hoping the fundraiser will spur more Thompson supporters — including those who may not want to publicly endorse McCain so soon after Thompson’s departure — to speak with their pocketbooks, if nothing else.

“I had intended to support Sen. McCain about the time that Fred Thompson was publicly toying with the idea of running,” said Tim Locke, of Smith-Free Group, who is now supporting McCain. “I’ll help, if nothing else with my checkbook.”

Other lobbyists are planning on doing more than just raising money.

Patrick O’Donnell, of Squire, Sanders & Dempsey, a former Thompson colleague at lobby shop O’Connor & Hannan, will be out on the trail for McCain. O’Donnell, who went to South Carolina in 2000 with McCain, says he let the campaign know he could travel to Florida over the weekend if necessary.

“I co-hosted a fundraiser here for [Thompson] and was working with anybody and everybody,” O’Donnell said of his efforts. “It’s too bad — I think he had all the basic ingredients.”

Rick Valentine, of K&L Gates, is also supporting McCain. Valentine had been Thompson’s leader of his social issues team. He’s already made contact with the recently formed Catholics for McCain effort that is being headed up by Sen. Sam Brownback (R-Kan.) and former Oklahoma Gov. Frank Keating, who now heads the American Council of Life Insurers.

But the decision isn’t easy for some longtime K Street lobbyists, who say they are worried about McCain’s hot temper and electability after his 2000 presidential bid.

Romney supporters also are trying to lure Thompson’s former backers.

“We got Morton Blackwell yesterday,” said David Norcross, of Blank Rome. “He was a very important pickup and I’m working on a couple of others.”

Blackwell, a conservative activist who runs the Leadership Institute, was a member of Thompson’s national committee from Virginia.

The Romney campaign also got a big boost when a group of Thompson lawyers, including former Justice Department assistant attorneys general Viet Dinh of Bancroft Associates and Rachel Brand endorsed the former Massachusetts governor last week.

Although they may not be among the most prodigious fundraisers, members of the group can provide a measure of support individually in Washington for the former governor.

Still, the Romney campaign doesn’t appear to have gotten as much traction among Thompson’s downtown supporters as it might have hoped.

Both camps also have looked to Capitol Hill for endorsements, hoping to use supportive members on the campaign trail as their campaign coffers are more and more tapped out.

Romney appears to be leading Congressional endorsements. Sen. Thad Cochran (R-Miss.) and Rep. Marsha Blackburn (R-Tenn.) both have endorsed Romney. Meanwhile, McCain received a boost after Florida Republican Rep. Jeff Miller moved over to his camp.

A few lobbyists suggested that they are taking the same tack as Thompson — not endorsing before Super Tuesday, hoping to ride out the current pressure to decide until the GOP has a nominee.

“I think people are going to wait,” said one downtown Thompson supporter. “They are not going back to testing the waters after investing so heavily for a year.”

Former U.S. Ambassador to Belgium Tom Korologos, a Thompson supporter, says he has long known both McCain and Romney, which makes the decision even harder.

“I have been kind of toying with either Romney or McCain,” Korologos said. No matter what he decides, Korologos said, he’ll probably give money to both candidates.

Some of the undecided K Street lobbyists are also some of the most sought-after fundraisers in town.

That list includes former Rep. Bob Livingston (R-La.), now of The Livingston Group, Chris Lamond of Ogilvy Government Relations and Eric Ueland of The Duberstein Group. Lamond, a former Thompson staffer, largely orchestrated Thompson’s downtown outreach effort for a July fundraiser that raised more than $250,000.

Likewise, Ueland, David Lugar of Quinn Gillespie & Associates, David Schwarz of Pier Strategies, longtime Thompson friend and campaign consultant Ken Reitz, and Covington & Burling’s Martin Gold haven’t endorsed either candidate.

“Both organizations have very sophisticated operations in place to very quickly reach out to those that have been otherwise endorsing someone else,” Lamond said.

He says he expects to make a decision shortly on which candidate he’ll support.