WEST LEBANON, New Hampshire (Reuters) - Republican presidential candidate Mitt Romney blasted U.S. President Barack Obama over the size of the national debt and the record number of Americans on food stamps, in a jab on the eve of a major speech by the Democrat.

Romney said "two big numbers out this week" prove that Americans are not better off than when Obama took office in 2009.

"We've gone from $10 trillion that the president inherited from all prior presidents to $16 trillion," Romney told reporters in the swing state of New Hampshire. The Treasury Department this week announced that the public debt had surpassed $16 trillion.

"The other number's forty-seven. Forty-seven million now on food stamps. When he came to office there were 32 million. He's added 15 million people," Romney said.

He was referring to an Agriculture Department report on Tuesday that showed the number of people on food stamps jumped to a record high of 46.7 million in June.

Romney has stayed mostly off the campaign trail this week, preparing for presidential debates while the Democrats hold their national convention in Charlotte, North Carolina.

The former Massachusetts governor said he did not watch first lady Michelle Obama's speech to the Democratic National Convention on Tuesday night, but said he thinks, "She's done a fine job as our first lady."

"I'm certainly not going to make any comments about the first lady's speech other than I respect her and she's a lovely person and a fine mom," Romney said.

Michelle Obama asked voters to give her husband four more years to fix the U.S. economy in an impassioned speech. Obama himself makes an address to the convention on Thursday night to accept the Democratic presidential nomination.

When asked how the debate preparation is going, Romney joked that Ohio Republican Senator Rob Portman, who is standing in as Obama during the mock debates, was a tough opponent.

"I'm just glad I won't be debating Rob Portman in the final debates. He's good," Romney said.

(Reporting by Sam Youngman, Editing by Alistair Bell and Doina Chiacu)