(CNN) Former Vice President Joe Biden's campaign on Wednesday argued it would be "nearly impossible for (Vermont Sen. Bernie) Sanders to recoup his current delegate disadvantage" in a campaign memo obtained by CNN.

In the memo, Biden's campaign states its internal count has his delegate lead at approximately 160, including between 70 and 80 netted in this Tuesday's primary. The Biden campaign's estimate does not match CNN's estimate, which has Biden's advantage at 142 delegates, with the former vice president at 803 and Sanders at 661.

The memo argues that next week's primaries in Arizona, Florida, Illinois and Ohio, and then the following week in Georgia, are in "some of our strongest" states and that the campaign expects to "significantly expand our delegate lead in those highly-supportive, delegate-rich states."

Biden's campaign also argues that Sanders' delegate gains in Vermont, Colorado and Utah pale in comparison to what Biden netted in Alabama, Mississippi and Virginia.

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"It's not just that Sanders has failed to win a large number of states, it's also that these wins have not netted significant delegates for him in the way they have for us," the memo says.

The memo later adds, "Should our broad base of support remain -- and we have seen no signs that would indicate otherwise -- it will be nearly impossible for Sanders to recoup his current delegate disadvantage."

The memo comes after Biden notched massive wins Tuesday in Michigan, Missouri, Mississippi and Idaho, and is in a close race with Sanders as Washington continues counting votes. The Vermont senator won only North Dakota among states contested on Tuesday.

As some Democratic officials and groups called Biden's delegate lead insurmountable, and the super PAC Priorities USA said it would begin backing Biden as the party's general election candidate, Sanders on Wednesday in Burlington, Vermont, pledged to continue in the race -- saying he would debate Biden on Sunday night in Arizona.

While speaking in Burlington, Sanders acknowledged that he's not doing well in the delegate count.

"Last night obviously was not a good night for our campaign from a delegate point of view," Sanders said, pointing out that he lost in Michigan, Mississippi, Missouri and Idaho.

But, he added, "What became even more apparent yesterday is that while we are currently losing the delegate count, approximately 800 delegates for Joe Biden and 660 for us, we are strongly winning in two enormously important areas which will determine the future of our country. Poll after poll, including exit polls show, that a strong majority of the American people support our progressive agenda."

The race is in the midst of a stunning two-week stretch in which Biden knocked out several leading competitors with his blowout win in the South Carolina primary, won 10 of the 15 contests on Super Tuesday and then scored a demoralizing victory Tuesday night in Michigan -- the state where Sanders had stunned Hillary Clinton four years earlier. The veteran Democrat has rapidly consolidated a coalition of African Americans, suburban voters and working-class whites. Sanders, meanwhile, has been stronger among young voters -- but those voters have turned out in smaller numbers.

"I want to thank Bernie Sanders and his supporters for their tireless energy and their passion," Biden said.

He echoed a common Sanders line on health care, touted their "common goal" and said they would work together to defeat President Donald Trump.

The Biden campaign memo states that attacks on the former vice president's record on Social Security and trade -- a key line of criticism from Sanders -- had "little impact," arguing Biden won union households and voters over 65 by wide margins in Michigan and Missouri.

The campaign also says it expected Tuesday to be the toughest priamry day in March.

The memo touts Biden's "broad coalition" and points to the suburbs and diverse counties -- St. Louis County, Missouri; Wayne County, Michigan, which houses Detroit; and Hinds County, Mississippi, which includes Jackson, in particular -- as places Biden "ran up the score."

"Turnout was up among the voters and in the states where we performed best. These are the kinds of states and voters who will deliver a victory in November," the memo states.