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 conservatives began competing to emerge as Mitt Romney’s chief rival.

Romney, the 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 their existing 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 even 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 primaries and caucuses that will pick a nominee to oppose President 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 ad’s implication, Gingrich and Santorum were out of Congress when the multibillion-dollar 2008 financial bailouts occurred. Paul and Bachmann voted against the legislation.

Still, the approach taken suggests that Perry is more concerned with outpacing Paul, Bachmann, Santorum and Ging-rich on caucus night that he is in defeating Romney.

Romney, making his second White House bid, has a well-funded and well-organized campaign nationally and in Iowa, as well as allies who are spending heavily on TV advertisements.

Others have risen to challenge him, but Romney has consistently stayed near the top.

A victory in Iowa does not necessarily translate into the Republican presidential nomination. Yet history suggests that contenders who finish farthest behind next week will quickly drop out, underscoring the significance of the struggle to emerge as Romney’s chief rival.

The most recent presidential hopeful to surge and then falter is Gingrich. His campaign imploded last summer and still shows the after-effects: a shortage of funds to counter attack ads in Iowa, and failure to qualify for the primary in Virginia.

After insisting he would run a purely positive campaign, Gingrich let it be known he was about to attack Romney.

Spokesman R.C. Hammond said Gingrich would make the case that Romney has advanced “very timid ideas that will do little to get people back to work.”

Gingrich favors an end to taxes on investment income and dividends, while Romney wants to end them only for individuals with incomes of $200,000 or less.

Gingrich also has proposed an optional 15 percent flat tax on income. Romney favors retaining the current graduated income tax system, with lower rates than currently exist.