Mitt Romney tightened his grip on the Republican presidential nomination after Rick Santorum's decision on Tuesday to leave the race, with a procession of Republican leaders and a key Santorum financier declaring their support.

Endorsements poured in on Tuesday as prominent Republicans said Santorum's exit signaled the need to unite behind Romney. Gov. Terry Branstad of Iowa, Gov. Rick Scott of Florida and Gov. Bobby Jindal of Louisana all announced they were backing Romney. The former Massachusetts governor also secured the backing of two leading conservative voices: Sen. Lindsey Graham of South Carolina and Sen. Pat Toomey of Pennsylvania.

It's time for all Republicans to focus their energies on the fall campaign, which will give Americans a fundamental choice between Obama's lurch toward European-style big government and the Republican alternative of a thriving private sector with a smaller government, Jindal said in a statement.

A leading Santorum donor also suggested that he would swing his support to Romney. Foster Friess, the retired investor who poured some $1.7 million into the pro-Santorum Super PAC the Red White and Blue Fund, told Politico that I'm obviously going to be of help in whatever way I can.

I've got some plans as to how I might be able to be of help, Friess added. The bottom line is, I'm going to be very supportive and I'll probably have plans to share with you a little later on.

Meanwhile, Newt Gingrich sought to claim Santorum's status as the conservative alternative to Romney, vowing to remain in the race until the Republican nominating convention in late August, and appealing to Santorum's supporters.

I humbly ask Senator Santorum's supporters to visit Newt.org to review my conservative record and join us as we bring these values to Tampa, Gingrich said in a statement after Santorum's announcement. We know well that only a conservative can protect life, defend the Constitution, restore jobs and growth and return to a balanced budget.

But more signs of distress emerged from Gingrich's flailing campaign, which is mired in $4.5 million of debt and recently cut back on staff. ABC reported that a check from the Gingrich campaign to Utah election officials bounced, potentially scuttling Gingrich's ability to appear on the ballot for the Utah primary.

The Obama campaign used the opportunity to sharpen its attacks on the man who will likely challenge President Obama in the fall.

It's no surprise that Mitt Romney finally was able to grind down his opponents under an avalanche of negative ads, Jim Messina, Obama's campaign manager, said in a statement. But neither he nor his special-interest allies will be able to buy the presidency with their negative attacks. The more the American people see of Mitt Romney, the less they like him and the less they trust him.