Mitt Romney, a potential running mate for John McCain, has joined the Republican shadow operation at the Democratic convention in Denver.

At a news conference, Romney charged that Barack Obama's policies would weaken America's economy and national security.

The former Massachusetts governor told reporters that Obama has a history of raising taxes and has voted to raise taxes or prevent tax decreases more than 90 times.

Romney compared Obama and McCain on spending, saying Obama constantly talks about raising spending. He said McCain is dedicated to reigning in excessive spending.