For a lot of Americans asking whether they're better off than when President Barack Obama took office in 2009, it's a tossup. Some things are better, such as job security, lending conditions and the stability of the overall economy. But there are fewer jobs, incomes have fallen and average net worth has plummeted. If things are better for the majority of Americans, it's only at the margins.

[Photos: Mitt Romney Accepts GOP Nomination]

But some Americans are indisputably better off--and the man who hopes to unseat Obama is one of them. As a member of the investor class, Republican presidential candidate Mitt Romney has benefited directly from one of the few developments under Obama that has been unambiguously positive. That means Romney is now running against policies that have helped make him even richer.

One thing Obama can genuinely crow about is a vigorous stock market rally during his first three-and-a-half years. From the day Obama took office, the S&P 500 stock index has risen by 64 percent. From a low point a couple of months after Obama's inauguration, in March 2009, it has soared by 110 percent.

A rising stock market helps most Americans, because it makes companies healthier and more likely to hire, helps build up pensions and retirement funds, and boosts overall confidence. But most of all, it helps stockholders, since their investments rise in value. And Mitt Romney's investments seem to have benefited as much as anybody's from the revived stock market.

[See who's better off under President Obama.]

Virtually all of Romney's income comes from investments. He hasn't detailed his holdings, which are managed in a blind trust, with no direct input from him. But the two sets of tax documents Romney has released make it pretty clear that Romney's wealth rises and falls with the stock market.

Romney's total earnings were about $22 million in 2010 and $21 million in 2011. A little more than half of that came from capital gains, with most of the rest coming from dividends and interest. Investors can earn capital gains on a variety of different assets, but stocks are one of the principal ones. And dividends usually are payouts that go to shareholders of public companies. So a significant portion of Romney's holdings are probably directly related to the direction of the stock market.

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There are also some indications in Romney's tax documents that he suffered major capital losses in 2008, a year in which the stock market plunged by 37 percent. The clue comes in Schedule D of his 2010 return, in which he claimed a $4.8 million loss carried over from prior years. "The carryover tells you that in prior years he had more losses than he could use on his tax return," says Brad Badertscher, an accounting professor at the University of Notre Dame. "The reason was probably the financial crisis."

[See who's worse off under President Obama.]

Stocks recovered in 2009, with the market up 23 percent for the year, and despite some dips, they've been heading up ever since. Overall, the rising stock market has helped Americans recover nearly $10 trillion worth of financial assets since 2008, according to Federal Reserve data. The bottom line for Romney is that he most likely suffered losses under Obama's predecessor, George W. Bush, then recovered under Obama, just like other investors.

Obama can't take all the credit. Federal Reserve policies, including quantitative easing, have been deliberately designed to reinflate the stock market, among other things. Bank bailouts initiated by Bush stabilized the industry that provides financing for the whole rest of the economy. And big companies represented on the stock markets have enacted many of their own measures to shed costs, eliminate vulnerabilities and remain profitable.

But even some of Obama's policies, such as temporary tax breaks for businesses, have helped wealthier Americans along with everybody else. So maybe Mitt Romney shouldn't complain too much about his opponent. Obama has been pretty good to him.

Rick Newman is the author of Rebounders: How Winners Pivot From Setback To Success. Follow him on Twitter: @rickjnewman.