* Differences to father could boost appeal
* Campaign role seen as move to gauge public opinion
* Sara Duterte says too soon to talk about presidency
* Daughter could protect father's legacy, ensure no legal action
* Duterte allies expected to dominate election
MANILA - During Philippine President Rodrigo Duterte's first two years in office, his daughter Sara had barely any interest in politics.
One year on, she is front and center in a midterm election that she isn't even running in, playing kingmaker for candidates allied with her father in what's being widely seen as a not-so-subtle trial balloon for her own presidential run in 2022.
Monday's elections are to a great extent a referendum on the Duterte administration, testing his popularity and giving him a chance to tighten his grip on power by retaining his Congressional majority, and keeping the opposition on the fringes of the all-important Senate for the remainder of his term.
Sara Duterte opted out of running for the Senate, choosing instead to manage the campaign of some of her father's loyalists, which experts say will boost her political capital and build alliances that could come in handy ahead of the next presidential election.
"She's projecting herself as a national personality. What's happening today is her testing the water," said Ramon Casiple, who heads the Institute for Political and Electoral Reform.
"Her image is being looked into, how people accept her. She has her own personality. She's not being looked at as a carbon copy of her father."
Sara Duterte reluctantly took over from her father as mayor of Davao City and has become hugely popular there. She is also no stranger to presidential events and overseas trips, serving as first lady because of her father's annulled marriage.
The 40-year old has spent the past three months on a campaign that has included touring on a 900 cc motorcycle and leading big rallies with billboards and banners carrying an image of herself twice the size of those of the candidates she is promoting.
Talk of succeeding President Duterte in 2022 has dampened concern among his critics that he might try to cling to power by changing the constitution to remove the single-term limit for presidents.
Casiple said Sara Duterte as president could protect her father's legacy and insulate him from political vendettas and what he has described as a "pattern of imprisonment" of former Philippine presidents.
Some critics even suggest a family succession would protect Duterte, 74, from a possible International Criminal Court (ICC) indictment for crimes against humanity. The ICC last year launched a preliminary examination into Duterte's war on drugs, in which police have killed thousands of people.
NO-NONSENSE, LESS DIVISIVE
For her part, Sara has urged her supporters to stop referring to her as the next president, insisting it would put her at the top of a "hate list" of people eyeing the job. Her intention, she said, was to help her father deliver on his agenda, and talk of succession was pointless until 2021.
She did not respond to questions sent by Reuters to her representatives.
Her appeal is that she has the same tough, no-nonsense approach that makes her father a hit among millions, but she is by no means his lackey.
She has been seen in public sharing lunch with opposition leader Leni Robredo, whose party the president despises.
She has openly disagreed with her father and some of his policies - though voiced support for his war on drugs - and has publicly insulted some of his inner circle.
She has also demonstrated that she's not to be crossed.
She once punched a court sheriff in the face, and her disdain for former house speaker Pantaleon Alvarez saw her bring key power-brokers together to oust him, on the same day of his annual state of the nation address to Congress.
Alvarez wants to win back his seat on Monday and declined to comment on Sara Duterte's political influence, or her backing of his rivals.
"I respect her politics," he said, when asked about her presidential potential.
According to some analysts, Sara's differences to her father could broaden her appeal among voters should she decide to run, allowing her to tap into groups alienated by his administration's perceived adversarial approach to civil society, the media and the intelligentsia.
"I like the way she's balancing out things when she openly contradicts blatantly ridiculous stuff that comes out of the administration," said Alan German, a political campaign strategist at Agents International in Manila.
"She's playing both side of the field. On the one hand she's got the Duterte supporters, obviously because of her last name and who she is, and on the other hand, she's trying to woo these sway guys who think 'it won't be so bad if it's Sara'," he said.
"She gives you just enough to think, OK she's not a lapdog."