With John McCain attacking him as a tax raiser, Sen. Barack Obama tied off the loose ends of his tax policies today, saying he would set the tax rates paid on most dividends and capital gains at 20 percent, substantially below where they stood during most of President Clinton’s presidency and lower than most Republicans expected.

He also clarified that that a proposal to impose Social Security taxes on incomes over $250,000 would not start until at least a decade from now.

The new details of the Obama tax plan are not new policies but are clarifications on vague statements from the senator from Illinois in the past. Obama has said for some time that he would raise dividends and capital gains tax rates from the 15-percent level reached in President Bush’s 2003 tax cuts, but he put the range somewhere between 20 percent and 28 percent. In an opinion piece in the Wall Street Journal today, Obama aides and advisers picked the lowest point in that range — a decision made as McCain pummels Obama as a tax hiker with a barrage of ads running during the Olympics.

By choosing a 20 percent tax, Obama is bringing tax rates on investments up to a mid-way point for families earning more than $250,000. Families below that level would continue paying the existing 15 percent rate. Obama economic aides Jason Furman and Austan Goolsbee noted that a 20 percent capital gains rate is well below the level set by Ronald Reagan in 1986. And Bush didn’t even think to lower the rate on dividends in his first round of tax cuts.