In the polling hierarchy, the least significant data measure is a president's personal popularity. Here, President Obama excels, with most polls showing him in the high 60s. Next comes his job approval, significant but not necessarily predictive.

Obama's approval, in the Rasmussen Poll, has now dipped to 51 percent, one point less than his 2008 vote share of 52 percent. In past polls, most voters registering disapproval for the president had voted for Sen. John McCain. Now, Obama's starting to lose people who backed him last November.

But the true predictive measurement is a chief executive's and his party's ratings on specific issues. As these shift, so usually do his job-approval numbers and eventually his popularity. And current trends suggest that Obama is in for rough sledding -- his job-approval ratings likely will quickly fall into negative territory and then drop further.

Rasmussen asked voters to compare which party was best on 10 issues. While Obama's ratings are likely better than his party's, the Republicans can take heart in trumping their opposition in eight of the 10 categories.

The most significant topic was, of course, the economy. For the second straight month, Rasmussen shows a GOP lead over the Democrats, this time by 46 percent to 41 percent, indicating that the incessant bad news and the collapse of the false hopes the stock market entertained this spring have taken their toll.

And only 39 percent of voters say that Obama is doing an excellent or good job on the economy, 11 points lower than his overall job approval. Forty-three percent say he's doing fair or poor.

As unemployment continues to rise, and even Obama predicts that times will get worse, this gap on economic issues will likely grow.

On their competing health-care reform plans, Rasmussen finds Obama and the Republicans drawing equal support. On health care generally, Democrats find their margin down to 4 points from 18 two months ago.

Obama is rapidly losing support on health reform, his key issue. And if he stays behind on health care and the economy for long, nothing much will hold him and his party aloft.

Rasmussen also found a Republican edge on many other issues. Democrats led Republicans 41 percent to 38 percent on education -- but the GOP led 49 percent to 40 percent on national security, 40 percent to 34 percent on immigration, 46 percent to 39 percent on abortion, 34 percent to 33 percent on ethics and corruption, and -- get this -- 42 percent to 37 percent on Social Security.

When Republicans are winning on Social Security, it's bad news for the Democrats.

Most ominous for Obama is the GOP lead on the economy, taxes and immigration, and the party's parity on health-care reform. The issues that will dominate the next few months are all working for the Republicans.

It's amazing how quickly Obama has lost his lead. His ratings had fallen steeply after his inauguration. Most polls had his job-approval dropping 13 points in his first three months. But then his numbers revived with the adulatory coverage at his first-100-days mark. Buoyed by hopes of a swift recovery, voters and the stock market gave him a break, and his ratings and the Dow rose sharply.

But since April, he has been dropping fast -- and his alarming losses on his central agenda issues portend further declines.

Congressional elections are still more than a year off, but Obama needs strong approval ratings to steer his legislative package through Congress. If he's sagging in the low 40s or high 30s by the fall, he probably won't be able to persuade moderate Democratic senators to walk the plank and vote for cap-and-trade, health-care reform, higher taxes or an immigration amnesty.

His chief legislative achievements, in other words, may already be behind him.