BOULDER, Colo. As President Barack Obama heads into the Democratic National Convention this week, he is seizing on the just-concluded Republican presidential convention to ramp up his re-election argument that Mitt Romney and his party are stuck in policies of the past and afraid to spell out the details of their plans.

“Despite all the challenges that we face in this new century, what they offered over those three days was an agenda that was better suited to the last century,” Obama told an estimated 13,000 people who filled a campus green Sunday at the University of Colorado against a scenic backdrop of the Rocky Mountains.

“It was a re-run. It could have been on ‘Nick at Night,“’ Obama said, referring to a popular cable television network that airs programs from the ‘50s and ‘60s. Viewers might as well have watched on a black-and-white TV with rabbit ear antennas, he joked.

And for all the Republicans’ talk of the hard choices they would make to address the country’s problems, “When Governor Romney finally had a chance to reveal the secret sauce, he did not offer a single new idea. It was just re-treads of the same old policies we’ve been hearing for decades, the same policies that have been sticking it to the middle class for years.”

Obama has spent the past few days on a “Road to Charlotte” tour, stumping in battleground states like Iowa and Colorado heading into the convention’s opening Tuesday at Time Warner Cable Arena in Charlotte.

Obama addresses a United Auto Workers Labor Day rally in Toledo before getting his first look at the aftermath of Hurricane Isaac in a stricken parish outside New Orleans. He's to meet emergency personnel who've been laboring since the storm hit last week to restore power and tend thousands of evacuees from flooded lands.

Romney was quick to issue a statement marking Labor Day as “a chance to celebrate the strong American work ethic.” But he added: “For far too many Americans, today is another day of worrying when their next paycheck will come.”

The post-Republican convention attacks on Romney and his party added a fresh element to Obama’s usual stump speech in which he describes Romney and congressional Republicans as backward-looking – to Bush-era fiscal policies and 20th-century social positions on issues like contraception, abortion rights and gay rights – and unwilling to spell out ideas like vouchers for future Medicare recipients.

Obama, in his speech in Boulder, decried Republican proposals for seeking more tax cuts for the wealthy, loosened financial and environmental regulations, and wanting to end to his health-care law expanding insurance coverage and benefits for existing policyholders. And he assailed their efforts to reverse clean energy and conservation measures.

Obama mostly described his own agenda as one of protecting the gains of the last four years, including on health care and increased college aid. For all his criticism of Romney, he faces a challenge this week in his own nomination-acceptance speech on Thursday night to outline more specifically what his second term would look like.

The president repeated his criticism, first lodged in Iowa on Saturday, that Romney “had nothing to say about Afghanistan” in his nomination acceptance speech.

Obama’s weekend visits to Iowa and Colorado marked the second time in a single week that he had campaigned in the two states. The fact that neither has many electoral-college votes – Colorado has nine, Iowa six – underscores just how close this presidential contest is believed to be.

And like his earlier trips to the states, Sunday’s venue was a college campus, reflecting Obama’s need to mobilize young voters much as he did four years ago. But this time he must do so with the handicap of an economy that leaves many of them without jobs or underemployed – as Romney points out routinely.

This was Obama’s 13th visit to Colorado as president, his eighth this year and his second to this liberal college town; in April, Obama spoke on his policies to make college more affordable, an issue he hit again on Sunday.

The state’s growing Latino population and its female voters have made Colorado friendlier turf for Democrats. But while Obama won handily here in 2008, the continued weak economy has made the state competitive for Republicans.

At the same time, the independent, libertarian strain among the state’s conservatives poses a challenge for Romney, as was reflected by the state’s delegation to the Republican convention.

The Denver Post reported that nearly a fourth of the delegation there was still supportive of Rep. Ron Paul, a libertarian and one of Romney’s vanquished rivals for the nomination.

The Colorado rally was Obama’s 47th since May, when he began holding reelection political events other than fundraisers, according to the tally kept by Mark Knoller, a longtime White House reporter for CBS News.

A breakdown of the swing states where he has held the rallies reflects the main fields of battle with Romney as the Obama campaign strategizes to reach 270 electoral votes. First is Iowa and second is Ohio, Obama’s next stop; his appearance on Monday in Toledo will be his 11th campaign rally in that state this year.

Next is Virginia, where Obama will hold his ninth rally on Tuesday before heading to Charlotte, N.C., Wednesday for his convention. Fourth in the number of campaign rallies is Colorado, which now has seen six, followed by Florida with five, New Hampshire with three, Nevada with two and Pennsylvania with one.

The Obama campaign, looking to build on any momentum coming out of their convention this week, on Sunday announced that upon leaving Charlotte Obama and his wife, Michelle, will travel with Vice President Joe Biden and his wife, Jill, to New Hampshire and Iowa on Friday. On the weekend they will separate, with Obama stumping in Florida and Biden in Ohio.

As tickets are distributed for each event, local Obama campaign organizers and volunteers collect information on attendees and see that those who are not registered – like students from other states – get registered. When early voting periods begin, they will contact those voters again to get their votes locked up before Nov. 6.

As Obama did here, in his speeches he seeks to close the deal by urging those in his audience to go to the websites gottaregister.com and gottavote.com for help.

After his event in Toledo on Monday, Obama will take a rare break from the campaign trail and fly to Louisiana, a solidly Republican state, to inspect the damage from Hurricane Isaac.

The Associated Press contributed.