DES MOINES, Iowa — An Iowa caucus campaign that has cycled through several GOP presidential front-runners entered its final week Monday, as unpredictable as the day hopefuls began competing to emerge as Mitt Romney's chief rival.

Romney, a former Massachusetts governor, released a new TV commercial for the state in which he cited a 'moral imperative for America to stop spending more money than we take in. It's killing jobs,' he said.

Texas Gov. Rick Perry countered with an advertisement that said four of his rivals combined — none of them Romney — have served 63 years in Congress, 'leaving us with debt, earmarks and bailouts.'

Former Pennsylvania Sen. Rick Santorum, who has invested more time in Iowa than any other contender, countered that 'most Americans now believe that a little bit of experience going into a job like president is probably a good thing.'

Santorum was the only presidential candidate in the state during the day. That changes today, with bus tours planned by Perry, former House Speaker Newt Gingrich and Minnesota Rep. Michele Bachmann, all eager to energize supporters and attract new ones. Texas Rep. Ron Paul arrives Wednesday. Recent polls suggest he is peaking as caucus day approaches, a rise that has him tied with Romney or ahead, and drawing more scrutiny for his views.

There were signs of strategic shifts as candidates struggled to stand out in advance of the straw poll next week that inaugurates the round of primaries and caucuses that will pick a nominee to oppose President Barack Obama next fall.

Perry's new ad shows images of Gingrich, Paul, Santorum and Bachmann as it criticizes Congress and renews the governor's call for halving lawmakers' pay and time spent in Washington. Despite the implication, Gingrich and Santorum were out of Congress when the financial bailouts of 2008 occurred. Paul and Bachmann voted against the legislation.