The United States imposed sanctions on Friday on Gebran Bassil, the leader of Lebanon's biggest Christian political bloc and son-in-law of President Michel Aoun, accusing him of corruption and ties to Hezbollah.
Bassil heads the Free Patriotic Movement (FPM), founded by Aoun, and has served as minister of telecoms, of energy and water and of foreign affairs.
Bassil, who has been the target of protests that erupted last year against a political class accused of pillaging the state, said in a Twitter post that sanctions did not scare him and that he had not been "tempted" by promises.
The sanctions could complicate efforts by Prime Minister-designate Saad al-Hariri, who is trying to navigate Lebanon's sectarian politics to assemble a cabinet to tackle a financial meltdown, Lebanon's worst crisis since its 1975-1990 civil war.
A source familiar with the process said the move was likely to harden the FPM's stance in negotiations on a new government needed to enact reforms demanded by foreign donors to tackle endemic corruption, waste and mismanagement to unlock aid.
In recent months, the United States has also placed sanctions on several officials linked to Hezbollah, the armed Iran-backed Shi'ite movement that has become Lebanon's most powerful political force, and which Washington considers a terrorist group.
The FPM has a political alliance with Hezbollah and Bassil has defended the group as vital to the defense of Lebanon.
The Treasury Department said Bassil was at the "forefront of corruption in Lebanon" where successive governments have failed to reduce mounting sovereign debt or address failing infrastructure and the loss-making power sector that cost state coffers billions of dollars while power cuts persisted.
"Through his corrupt activities, Bassil has also undermined good governance and contributed to the prevailing system of corruption and political patronage that plagues Lebanon, which has aided and abetted Hizballah’s (Hezbollah) destabilizing activities," US Secretary of State Mike Pompeo said in a statement.
A senior US official said Bassil's support for Hezbollah was "every bit of the motivation" for targeting him for sanctions.
Bassil was sanctioned under the Global Magnitsky Human Rights Accountability Act, which targets human rights abuses and corruption around the world. It calls for a freeze on any US assets and prohibits Americans from doing business with him.
The State Department also imposed a ban on Bassil's travel to the United States.
A senior US official said the sanctions announcement was "not intended to impact a government formation process" in Lebanon. The official also denied any connection between the announcement and this week's US elections, saying such sanctions packages take months to prepare.