Romney denies clash with running mate as he refuses to say whether he supports plan to reshape the healthcare programme

Mitt Romney's presidential campaign has been placed on the defensive over proposals by his running mate Paul Ryan to reshape Medicare, the popular healthcare programme for over-65s that has become a hot election issue.

The Democrats stepped up their campaign on an argument they think is a winner, sending spokesmen out to claim Ryan's plan to reform the system would spell the end of Medicare, and distributing 100,000 bumper stickers that read: "Save Medicare, Vote Democrat".

Since the announcement of Ryan as Romney's vice-presidential pick, the Republican challenger has faced persistent questioning over where he stands. The Romney team has been left vulnerable, in part because it has been sending out mixed messages.

Romney, in a rare press conference on Monday night in Florida, repeatedly refused to say whether he backed Ryan's Medicare reform plan. Some of advisers have gone on television to say publicly that he wholly and enthusiastically endorsed Ryan's budget proposals and would, if president, have signed it. Others have sought to distance him from it, saying Romney was running on his own plan.

At the Miami press conference, Romney insisted there was no contradiction. "I'm sure there are places that my budget is different than his, but we're on the same page," Romney said. "My plan for Medicare is very similar to his plan for Medicare."

Barack Obama's senior campaign adviser, David Axelrod, returned to the television studios on Tuesday to say that election of the Romney-Ryan ticket would lead to a death spiral for Medicare.

The issue is likely to dominate Ryan's visit at the weekend to Florida, a swing state in which seniors make up a large part of the population and where the Republican convention is to be held just over a week later.

Republicans are hoping that the convention in Tampa will energise the campaign and silence anonymous critics within the party. On Tuesday, Politico published a lengthy piece, quoting unnamed party activists who said there was concern amongst GOP operatives in Washington about the Ryan pick, and in particular the Medicare issue. "This could be the defining moment of the campaign. If they win the battle to define Medicare, then I believe Romney wins the presidency. If they lose it, then they lose big in the fall," said one unnamed "strategist" quoted by Politico.

On Tuesday, the Republicans announced that Chris Christie, the charismatic New Jersey governor, who was touted as a possible vice-presidential candidate, is to be given a prominent role, making the keynote address. The Florida senator, Marco Rubio, who Romney confirmed had been on his vice-presidential shortlist, is also being given a prominent slot, introducing Romney when he makes his speech accepting the party nomination as Republican presidential candidate.

The keynote address at the Democratic convention will be given by former president Bill Clinton.

Ryan is campaigning Tuesday in Nevada where a pro-Obama Super Pac funded mainly by unions is sponsoring an advertising campaign saying seniors are at risk from the Romney-Ryan ticket. Ryan is scheduled for a fundraising event at a casino complex owned by Sheldon Adelson, one of the big contributors to the Republican party.

Adelson is a divisive figure: the New York Times reported Tuesday that a Chinese businessman linked to Adelson is the focus of a federal investigation into potential bribery.

• This article was amended on 15 August 2012. The original referred to Marco Rubio as the Florida governor, rather than senator. This has been corrected.