He has frequently rebelled against his party, specifically in his calling for a referendum on the European Union (EU) before the 2015 election and in opposing military intervention in Iraq, Libya, and Syria.

[15] Baron was a member of Iain Duncan Smith's frontbench team, but resigned in March 2003 in protest at his support of the Iraq War.

[16] He was re-appointed by Duncan Smith as a health spokesman four months later,[17] a position he held until July 2007 when he was moved to the Conservative Whip's Office.

In 2013 he tabled a backbench motion to mandate a vote in Parliament before providing "lethal support" to anti-government forces in Syria, which ultimately prevented further military intervention when the government was unable to secure the necessary parliamentary support[23] and he was also part of a minority voting against the government on air strikes against Islamic State of Iraq and the Levant.

[24] In June 2012, Baron delivered a letter, signed by over 100 Tory MPs, to the Prime Minister David Cameron urging him "to place on the Statute Book before the next General Election a commitment to hold a referendum during the next Parliament on the nature of our relationship with the European Union".

[25] In May 2013 he tabled a rebel amendment to the Queen's Speech to "express regret" that a referendum on the EU could not be held sooner, which was backed by over 100 MPs.

Speaking to the BBC's Newsnight in response to speculation in late 2014, Baron said: "You should never say never in politics, but the bottom line is my very strong preference is to stay within the Conservative party.

"[23] At the 2015 general election, Baron was re-elected as MP for Basildon and Billericay with a decreased vote share of 52.7% and an increased majority of 12,482.