Mitt Romney is enjoying a burst of energy after adding Rep. Paul Ryan to the Republican presidential ticket. He is drawing the biggest and most enthusiastic crowds of his campaign, the same way GOP nominee John McCain did four years ago after naming Sarah Palin as his running mate. Romney is getting what he hoped for when he passed over safer choices.

But he also has bought trouble, as is clear from Democrats’ attacks on Ryan’s far-reaching and controversial budget plan, which would, among other things, transform Medicare into a premium support program for younger people upon retirement.

Whether or not Romney wanted a debate about Medicare, an issue that long has favored Democrats, he has one. His campaign advisers recognize the dangers. From their perspective, it’s better to have the discussion now than in October. They are trying to take this fight to the president in a way that no Republican nominee has done before.

On Tuesday, the Romney campaign began its counterattack on the Medicare issue even before President Obama’s campaign could air its first ad on the subject. Romney’s ad charges that Obama cut more than $700 billion from Medicare to help finance his controversial health-care overhaul.

“We’re the ones who are offering a plan to save Medicare, to protect Medicare, to strengthen Medicare,” Ryan (Wis.) told Brit Hume of Fox News Channel. “President Obama is actually damaging Medicare for current seniors. It’s irrefutable. And that’s why I think this is a debate we want to have, and that’s a debate we’re going to win.”

Romney is dealing with two problems: the details of Ryan’s budget blueprint, and questions about the differences between the running mates’ fiscal and Medicare plans.

Romney and his advisers insist that he will run on his plan, not Ryan’s. In part, they’ve done that to remind people that the tail will not wag the dog, that the running mate will not overshadow the nominee. Any presidential candidate would say the same thing.

But keeping Ryan’s plan out of the debate is virtually impossible. Romney embraced the conceptual framework of the congressman’s blueprint long before he selected Ryan as his running mate. At the time, he could preserve some space to say he wouldn’t follow every detail of Ryan’s plan.

That was before he put on the ticket a politician described as the intellectual leader of the GOP, who has been in the thick of the battle over how to transform government through tax cuts, budget reductions and entitlement reform. Pick Ryan, and you get his blueprint as your own.

On the big issues, Romney and Ryan are in agreement. They favor big tax cuts from which the wealthiest Americans would benefit significantly. They have not fully explained how they would offset that lost revenue. They support reductions in domestic discretionary spending. Both want changes that would convert Medicare into a premium support program for younger workers. Their priorities are the same.

Romney hasn’t said whether he has real differences with Ryan or mostly minor ones — on Medicare or anything else in the budget proposal. The last thing he wants is a Romney-Ryan debate, but if there are substantive differences, they ought to be highlighted and explained. One real difference is that Ryan accepts the cuts Obama made to Medicare as part of his budget. Romney would restore them but hasn’t explained why he objects to what Ryan would do.

Romney hoped that the choice of Ryan would amplify his message that the status quo or even small changes aren’t going to solve the country’s fiscal problems. That is a big argument and a debate worth having. Right now, however, Romney is dealing with questions about whether Ryan’s plan would hurt seniors, the middle class or the poor.

Democrats are seizing the moment. Obama is traveling across Iowa this week trying to tie Romney via Ryan to congressional Republicans, whose favorability rating is in the basement. Vice President Biden is attacking Ryan almost as if he were the nominee.

Obama campaign advisers are brushing aside any idea that there is daylight between Romney and Ryan and focusing on Ryan’s budget for what is likely to be a campaign of negative ads. The Democrats are using August as they used July, to try to define the opposition before Romney — and now Ryan — can fully defend and define themselves.

Romney’s campaign advisers believe they have opportunities to win this debate. Obama’s economic record remains the biggest threat to his reelection bid. He is vulnerable as well to the criticism that he is not offering real leadership on entitlement reform. The new Medicare ad seeks to exploit what the president did to Medicare to finance his health-care program and put Democrats on the defensive.

Ironically, Democrats cried foul over the new ad, saying Obama was cutting the rate of growth in the program, not reducing actual spending. That ignores the fact that, in the 1996 campaign, Democrats attacked Republicans for cutting Medicare spending when Republicans were reducing the rate of growth in the program.

The Republican National Convention will give Romney a chance to tie everything together: his biography presented in its most positive way; the policy differences with Obama outlined with clarity; the economic and fiscal arguments advanced with sharpness and elevation; and the Obama attacks rebutted cleanly. The campaign may look and feel different at that point.

But Romney and Ryan face the possibility that, before the convention, Obama and the Democrats will define Ryan’s budget — and in particular his changes to Medicare — so negatively that the damage will be long-lasting. That’s why Romney’s campaign has moved quickly to blunt the Medicare attacks. But this fight is just starting, which is what makes these weeks a defining moment in the campaign.

Read previous Take columns at