President Obama is no longer the underdog.

A wave of new polls makes the point: Obama would defeat each of his four Republican challengers by substantial margins, and the trends are increasingly favorable for his re-election. The latest CBS News-New York Times poll, for example, finds that Obama would defeat former Sen. Rick Santorum of Pennsylvania, 49 percent to 41 percent; he would beat former Gov. Mitt Romney of Massachusetts, 48 to 42; he would vanquish Rep. Ron Paul of Texas, 50-39, and demolish former House Speaker Newt Gingrich, 54-36.

This turnaround is expected to change the dynamic of the presidential race in several ways.

First, it is likely to boost the morale of Democrats, increase their enthusiasm, and help Obama and his allies raise more money.

Second, it will have a deflating effect on Republican morale after many months when GOP leaders and voters thought that toppling Obama would be very likely.

Third, it will encourage Obama to proceed with the populist approach he has been taking, hammering the wealthy and big corporations for not paying their "fair share" in taxes and pledging to defend the middle class.

Finally, Obama's surge is expected to push the Republicans to attack him even more harshly in an effort to slow his momentum.

The polls that have Obama ahead nationally and in most of the key swing states include surveys by CBS News-New York Times, CNN-Opinion Research, the Pew Research Center, and Fox News.

Among the reasons given by Democratic and Republican strategists for the Obama turnaround: Public perceptions that the economy is improving; a gradual decline in the unemployment rate; the sense among many voters that the Republicans don't offer superior answers to Obama's; Obama's winning the message war against congressional Republicans by portraying them as obstructionists and defenders of the powerful, and the negativity of the GOP presidential candidates, which turns off many independent voters.