Detroit (CNN) Hillary Clinton will not debate Bernie Sanders in California, her top campaign spokeswoman said Monday.

Declining to participate in the Fox News debate -- which Sanders had agreed to -- is another clear sign that Clinton and her top aides are fully focused on the general election against Donald Trump, the presumptive Republican nominee.

"As we have said previously, we plan to compete hard in the remaining primary states, particularly California, while turning our attention to the threat a Donald Trump presidency poses," Jennifer Palmieri, Clinton's spokeswoman, said. "We believe that Hillary Clinton's time is best spent campaigning and meeting directly with voters across California and preparing for a general election campaign that will ensure the White House remains in Democratic hands."

Clinton's campaign is currently fighting a two-front war against both Trump, who has all but locked up the nomination, and Sanders, who has pledged to campaign until the last vote is counted in June.

"I will be the nominee for my party," Clinton told CNN last week.

JUST WATCHED Reporter: Sanders wants say over Clinton cabinet picks Replay More Videos ... MUST WATCH Reporter: Sanders wants say over Clinton cabinet picks 06:02

Clinton and Sanders, however, agreed in February to add four more debates to the calendar, including a debate in May. So far, the two candidates have had three of the four debates the agreed to, the last being in New York on CNN.

Sanders, behind in the popular vote and in delegates, agreed to an invite from Fox News late last week. A statement from the Sanders campaign Monday night said Clinton "reneged" on her prior debate agreement.

"I am disappointed but not surprised by Secretary Clinton's unwillingness to debate before the largest and most important primary in the presidential nominating process," Sanders said.

"I also would suggest that Secretary Clinton may want to be not quite so presumptuous about thinking that she is a certain winner," Sanders added. "In the last several weeks, the people of Indiana, West Virginia and Oregon have suggested otherwise."

Fox News formally proposed the debate last Tuesday, though the network always knew it was a stretch for a number of reasons. Political considerations aside, Clinton's appearances on the network are few and far between.

JUST WATCHED Sanders rejects notion he is damaging Clinton against Trump Replay More Videos ... MUST WATCH Sanders rejects notion he is damaging Clinton against Trump 01:33

Fox's proposal letter noted: "Democrats have not debated on Fox News for the last 12 years."

But the pitch went like this: Clinton and Sanders "will reach a vast number of viewers -- many of whom are certainly open to persuasion -- just as we pivot to the general election."

In other words, it was promoted as a way to appeal to moderates.

For the Clinton campaign, the proposal was a non-starter. There was no advantageous reason to debate, and if there was, other networks would be in the running to host it.

Bill Sammon, FOX VP and Washington managing editor, said in a statement, "Naturally, Fox News is disappointed that Secretary Clinton has declined our debate invitation, especially given that the race is still contested and she had previously agreed to a final debate before the California primary."

But now Fox's executives can say they tried -- and its conservative-leaning hosts and commentators can criticize Clinton for the snub. Time will tell if any do.

Anchor Bret Baier straightforwardly reported Clinton's decision on his nightly political newscast on Monday night.

"The decision, they say, has nothing to do with Fox," he said.

Baier's panelists said they thought Clinton's decision was politically logical.

But it does set a not-so-friendly tone between Clinton and the conservative-themed network as the general election looms.

Clinton has largely tried to avoid talking about the potential California debate. When she was asked about it on Sunday, she told NBC that she had not thought about it.

"We'll consider it," Clinton said. "But I think what's important is we're not going to let, at least my campaign, is not going to let Donald Trump try to normalize himself" when he doesn't have an opponent.