Democratic presidential candidate Barack Obama held on to his post-convention lead over John McCain, as Republicans gathered for the second day of their national convention in St. Paul, Minn.

Obama, a senator from Illinois, has a lead of just under 7 percentage points in an average of five national polls taken since the Democratic National Convention ended and McCain announced Alaska Gov. Sarah Palin as his running mate, according to figures compiled by the Web site Realclearpolitics.com.

Less than 2 points separated the two candidates before last week's Democratic convention in Denver, which ended Aug. 28.

Candidates historically get a boost in poll ratings following their party conventions as voters pay more attention to the campaigns. The Republican convention ends Thursday, after McCain formally accepts the party's presidential nomination.

In a Gallup Inc. tracking poll covering the period of Aug. 30 to Sept. 1, Obama for the first time hit the 50 percent mark for public support in the poll, according to figures posted on the firm's Web site. He led McCain by 8 percentage points.

Obama also was ahead in a daily tracking poll conducted by Rasmussen Reports, which showed him with 48 percent support to 43 percent for McCain, a senator from Arizona.

A CBS News poll, which includes the vice presidential candidates, showed the Democratic ticket with 48 percent to 40 percent for the Republican candidates. In a CBS poll taken before Obama named Delaware Sen. Joe Biden as his running mate and McCain picked Palin, Obama led by 3 percentage points.

The poll showed that public opinion about the vice presidential candidates is still forming.

While 37 percent of those surveyed said they have a favorable view of Biden, a six-term U.S. senator, 47 percent are undecided or don't know enough about him to have an opinion. Palin was viewed favorably by 22 percent in the CBS poll and two-thirds were undecided or didn't know enough about her.

A poll conducted by American Research Group Aug. 30 to Sept. 1 showed Obama leading McCain 49 percent to 43 percent. Another survey conducted for the National Journal's Hotline publication showed Obama leading 48 percent to McCain's 39 percent. Before the convention Obama had a 4-point edge in the Hotline/Diageo survey.

Of the six most recent polls, only one showed a closer race. A CNN poll conducted Aug. 29-31 put the contest at a tie.

The three-day Gallup poll surveyed about 1,000 people each day with the results combined into an average. The margin of error is plus or minus 2 percentage points. The Rasmussen survey interviewed 1,000 likely voters each night over three nights and has a margin of error of 2 percentage points.

CBS surveyed 781 registered voters and the poll has an error margin of 4 percentage points. The American Research Group poll interviewed 1,200 likely voters Aug. 30-Sept. 1 and has a margin of error of 3 percentage points.

The Hotline/Diageo poll contacted 805 registered voters Aug. 29-31 and had a 3.5 percentage point margin of error.