Republican presidential candidates are all but silent on Senate GOP leader Mitch McConnell's proposal to sidestep a potentially disastrous government default on loan obligations.

Not one candidate has endorsed it, and Rep. Michele Bachmann of Minnesota and Newt Gingrich have come out against it. The former House speaker tweeted: "McConnell's plan is an irresponsible surrender to big government, big deficits and continued overspending."

The hesitation to weigh in _ by front-runner Mitt Romney and his rivals _ underscores the challenging politics at play for GOP White House hopefuls in the fast-moving debate over increasing the United States' borrowing ability.

Most aren't in Congress, don't have a vote, and, thus, seemingly feel little obligation to wade into the specifics of the complex debate. When they do speak, they are being careful about what they say on the issue, perhaps mindful that whoever emerges as the eventual 2012 nominee _ and party standard-bearer _ will likely have to contend with the result of tense negotiations between the White House and Congress.

The Treasury Department says lawmakers have until Aug. 2 to extend the nation's debt limit to prevent a catastrophic government default on its bills.

McConnell, a Kentucky Republican, put the candidates on the spot Tuesday when he offered a proposal that guarantees President Barack Obama's requests for new government borrowing authority unless Congress musters veto-proof two-thirds majorities to deny him. Conservatives assailed the plan that quickly became the latest litmus test for the party's base.

Democrats tried to use it to their advantage, signaling that they intend to try to make the debt debate an issue in the 2012 election.

"Anyone heard from Mitt Romney lately? Where is he on McConnell plan? On the debt talks? On the impact of a default? Why so quiet?" former Obama senior adviser David Axelrod tweeted Wednesday, tweaking the former Massachusetts governor.

Romney adviser Eric Fehrnstrom sidestepped an opportunity to answer those questions and instead whacked Obama back on Twitter, saying: "I have a question for (at)davidaxelrod: Where are the jobs? We're not just on wrong track; it feels like we're tied to the tracks."

Pressed on where Romney stood on McConnell's plan, aides repeated Romney's oft-spoken stance on the debt discussion: "A vote on raising the debt ceiling has to be accompanied by a major effort to restructure and reduce the size of government." He deferred to Congress on the details of any such plan.

He and most others say they are open to increasing the nation's borrowing authority but they are insisting that any increase is coupled with provisions that are designed to endear them to the party's conservative base, including a constitutional amendment that would insist on a balanced federal budget and spending cuts.

"We cannot allow it to increase without game-changing reforms to our federal budget. Let's start with a balanced budget amendment, a cap on federal spending levels and real cuts to this year's budget," former Minnesota Gov. Tim Pawlenty, who supports the increase if it allows the GOP to accomplish other political goals, said in an opinion piece published Tuesday in The Des Moines Register.

Gingrich, the former House speaker, has a similar position, saying: "They should pass a spending cut, a savings bill and a balanced budget amendment and dare President Obama to veto them."

Two other candidates _ who unlike the others would vote on any plan _ have split from the rest, promising to vote against a debt-limit increase.

"Republicans cannot take the bait and get fooled again," said Rep. Ron Paul of Texas. And Bachmann, the tea party favorite, noted the issue in her first presidential campaign ad, saying: "I will not vote to increase the debt ceiling."

On Wednesday, she and two other congressional conservatives proposed a bill to guarantee military personnel and others would get paid if Obama and congressional Republicans cannot agree to raise the nation's debt limit.

When asked if she would vote for McConnell's last-ditch effort, Bachmann said she would vote "no."

"I've been here long enough that I've seen a lot of smoke and mirrors but I haven't been here long enough to forget who I serve or where I come from," she told reporters. "And again all I can reiterate is that people across America are saying the spending is what has to be addressed, it's too much, it's got to be limited."

Former Sen. Rick Santorum of Pennsylvania scolded those who oppose any deal, saying while campaigning in South Carolina that "those who say we should not have any debt limit increase and solve our problem now, that is not sound decision-making."

One other candidate campaigning in South Carolina this week, former Utah Gov. Jon Huntsman, largely took a pass on specific issues in the debate. He supports a balanced budget amendment but sidestepped details of the negotiations, saying "Let it play out. There's time remaining."

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Associated Press writers Jim Davenport in Greer, S.C., and Bruce Smith in Charleston, S.C., contributed to this report.