WASHINGTON — President Bush's campaign strategists say they are planning to attack Sen. John Kerry's running mate as a second choice no matter who it turns out to be. They say Kerry has made clear that his first choice was a Republican who still stands at Bush's side, Sen. John McCain.

"We think it's important that people understand that this is a ticket of John Kerry and his second choice," Nicolle Devenish, the Bush campaign's communications director, said.

The effort to turn Kerry's flirtation with McCain against him — an effort that Democrats expect will include a commercial featuring a recent speech by McCain praising the president — is part of a multipronged strategy to offset what the Bush campaign assumes will be a sharp swing in the polls for Kerry. That, Bush's aides say, will come in the wake of Kerry's announcement of a running mate and his acceptance of the Democratic nomination in Boston at the end of this month.

Matthew Dowd, Bush's chief campaign strategist, said he expects the race to shift from dead even now to as much as a 15-point advantage in national polls for Kerry by the end of the Democratic convention.

Though campaigns typically seek to set expectations as low as possible and Kerry's aides have said they do not expect any such lead, Dowd's public forecasts have tended to be relatively accurate. He said his prediction is based on the average upward bounce in the polls for challengers against incumbents over the last three decades at this stage in the race.

To try to make sure that any surge by Kerry is temporary, Republicans who discuss campaign strategy with the White House said they expect Bush to begin a heavy wave of critical advertising against the Massachusetts Democrat and his running mate come August.

Democratic officials said they were prepared for Bush to run a commercial featuring McCain alongside the president, perhaps even on the day Kerry announces his vice presidential selection.

Kerry's aides say that Bush will be able to make only so much of McCain's support, noting that McCain went out of his way to defend Kerry's national security credentials amid attacks by the Bush campaign during the spring.