"I chose to run proudly in the Democratic primary and caucus and look forward to winning that process. But clearly, as a nation, I think we flourish when there are different ideas out there," Sanders said during MSNBC's Democratic presidential candidate forum in Nevada on Thursday.

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"Sometimes the two-party system makes it very, very difficult to get on the ballot if you are a third party, and I think that's wrong. I think we should welcome competition."

Sanders is the longest-serving Independent in Congress, though he has caucused with the Democrats throughout his time in Congress. He decided to run for president in the Democratic contest.

His lack of history as a Democrat has prompted criticism from his rivals for not embracing the party sooner.

The candidates spoke separately at the forum two days before the caucuses in Nevada, where polls show Sanders is essentially tied with Clinton after she had held a large lead throughout most of the race.