"In recent years, democracy in Hungary has significantly eroded. ... Under Orban, the election process has become less competitive and the judiciary is increasingly controlled by the state," Republican Senate Foreign Relations Committee Chairman Jim Risch and Bob Menendez, the panel's top Democrat, said in a letter to Trump.
The letter was also signed by Republican Senator Marco Rubio and Democratic Senator Jeanne Shaheen.
Several Democratic members of the House of Representatives, led by House Foreign Affairs Committee Chairman Eliot Engel, called on Trump to cancel his meeting with Orban, citing similar concerns as well as his anti-Semitic and xenophobic comments.
Orban is a nationalist leader who has often had conflicts with the European Union over his anti-immigration campaigns and judicial reforms. He clashed with the administration of then-U.S. President Barack Obama, a Democrat, over what critics said was an erosion of democratic values by his government.
Trump is due to meet with Orban on Monday.
The senators also said they were very concerned about the close relationship between Hungary, a NATO partner, and Russia. They said Hungary has failed to diversify its energy resources from Moscow and allowed Russia to exploit its visa system to evade U.S. sanctions.