President Obama thanked his Iowa supporters Tuesday evening and warned them to prepare for a difficult reelection fight against the eventual Republican nominee in the months to come.

Obama is without a serious primary rival. But on an evening when attention was focused on the Republican contest, he chose to address Iowa Democrats, who set him on his way to the presidency four years ago, to remind them how much he will need their help to secure a second term at a time of high joblessness, economic despair and political strife.

Speaking from the Capitol Hilton in Washington, Obama told his party’s caucus participants through videoconference that “because of you, I have never lost that sense of inspiration that drove me to embark on this journey in the first place.”

“What you guys are doing at the Democratic caucus and what you’ll be doing every day until November will make the difference,” Obama said.

It was his decisive win in Iowa four years ago that gave an unproven campaign instant credibility with Democratic primary voters, a critical first step on the rough road to the nomination. In his victory remarks then, Obama reveled in what he characterized as an outsider’s win against an establishment he promised to change once in office.

“They said our sights were set too high,” Obama said in that brief speech, which campaign aides evoked Tuesday for the pledges it contained to end the Iraq war, secure health insurance for all Americans, fight climate change and reduce dependence on foreign oil. “They said this country was too divided, too disillusioned to ever come together around a common purpose.”

His campaign purchased a high-profile advertisement that ran Tuesday on the Des Moines Register’s homepage, showcasing those pledges and indicating that each had been accomplished.

But the nation’s politics have grown even more split along partisan lines and acrimonious in tone over the past four years, leaving many less enthusiastic about the president who promised unity and change.

On Tuesday, Obama remained hopeful.

“In some ways, I’m actually more optimistic now than when I first ran,” Obama said in response to a question from a caucus participant about whether he still believes in “hope and change.” “We’ve already seen change take place. 2012 is about reminding the American people how far we’ve traveled.”

Asked by a voter named Carol White in Cedar Rapids how he answers criticism that he hasn’t done enough, Obama said: “We’ve done a lot, and we have a lot more to do. That’s why we need four more years.”

Obama’s approval rating sits at 45 percent, according to the most recent Gallup survey. That marks an improvement from a low of 38 percent in mid-October but is still worrisome to Obama supporters as a difficult election year begins. Obama also faces the political challenge of remaining in front of voters during the months it may take Republicans to select a nominee.

His decision to address Iowa supporters served as a way to edge into the frame of a presidential race dominated for now by Republicans, while managing to stay above the opposition party’s bitter primary fight.

He also has a large press corps and even larger plane at his disposal, which he will use Wednesday to travel to Ohio, a swing state, to deliver a speech on the economy in Cleveland. Such events have recently had the feel and tone of campaign rallies.

His press secretary, Jay Carney, told reporters Tuesday that Obama will describe his efforts “to grow the economy and create jobs, to protect the middle class, to expand it and to make the middle class more accessible to those who aspire to it.”

“And going back to Iowa four years ago, that was his number one focus then,” Carney said.

Obama won Iowa in the 2008 general election, as well, and his reelection campaign is working hard to repeat the victory.

The campaign has opened eight field offices in Iowa, which will probably remain in play until late in the campaign, and held many events to keep voter enthusiasm up.

In reflecting on his achievements, Obama also warned that Republicans will seek to “roll back regulations on clean air and Wall Street reform.” He enlisted supporters’ help in defeating those efforts and the broader one to vote him out of office. “It’s going to be a big battle, though,” he said. “I hope you guys are geared up. I’m excited.”